Temperature gradients for controlling memristor switching

ABSTRACT

A memristor includes a bottom electrode, a top electrode, and an active region disposed therebetween. The active region has an electrically conducting filament in an electrically insulating medium, extending between the bottom electrode and the top electrode. The memristor further includes a temperature gradient element for controlling switching.

BACKGROUND

Memristors are devices that can be programmed to different resistivestates by applying a programming energy, such as a voltage. Afterprogramming, the state of the memristor can be read and remains stableover a specified time period. Thus, memristors can be used to storedigital data. For example, a high resistance state can represent adigital “0” and a low resistance state can represent a digital “1” (orvice versa). Large crossbar arrays of memristive elements can be used ina variety of applications, including random access memory, non-volatilesolid state memory, programmable logic, signal processing controlsystems, pattern recognition, and other applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B depict a set process (FIG. 1A) and a reset process (FIG. 1B)for a memristor having an axial thermal gradient, according to anexample.

FIGS. 2A-2B depict a set process (FIG. 2A) and a reset process (FIG. 2B)for a memristor having a radial thermal gradient, according to anexample.

FIG. 3 depicts a method for changing a state of a memristor using athermal gradient, according to an example.

FIG. 4 depicts a method for manufacturing a memristor having atemperature gradient element for providing a thermal gradient, accordingto an example.

FIG. 5 depicts an alternate method for manufacturing a memristor havinga temperature gradient element for providing a thermal gradient,according to an example.

DETAILED DESCRIPTION

It is to be appreciated that, in the following description, numerousspecific details are set forth to provide a thorough understanding ofthe examples. However, it is appreciated that the examples may bepracticed without limitation to these specific details. In otherinstances, well-known methods and structures may not be described indetail to avoid unnecessarily obscuring the description of the examples.Also, the examples may be used in combination with each other.

While a limited number of examples have been disclosed, it should beunderstood that there are numerous modifications and variationstherefrom. Similar or equal elements in the Figures may be indicatedusing the same numeral.

It is be noted that, as used in this specification and the appendedclaims, the singular forms “a,” “an,” and “the” include plural referentsunless the context clearly dictates otherwise.

Memristors are nano-scale devices that may be used as a component in awide range of electronic circuits, such as memories, switches, radiofrequency circuits, and logic circuits and systems. In a memorystructure, a crossbar array of memristor devices may be used. When usedas a basis for memories, memristors may be used to store bits ofinformation, 1 or 0. When used as a logic circuit, a memristor may beemployed as configuration bits and switches in a logic circuit thatresembles a Field Programmable Gate Array, or may be the basis for awired-logic Programmable Logic Array. It is also possible to usememristors capable of multi-state or analog behavior for these and otherapplications.

The resistance of a memristor may be changed by applying a voltageacross or a current through the memristor. Generally, at least onechannel may be formed that is capable of being switched between twostates—one in which the channel forms an electrically conductive path(“ON”) and one in which the channel forms a less conductive path(“OFF”). In some cases, conducting channels may be formed by ions and/orvacancies. Some memristors exhibit bipolar switching, where applying avoltage of one polarity may switch the state of the memristor and whereapplying a voltage of the opposite polarity may switch back to theoriginal state. Alternatively, memristors may exhibit unipolarswitching, where switching is performed, for example, by applyingdifferent voltages of the same polarity.

Examples disclosed herein provide for thermally-insulated memristordevices. In example implementations, a memristor device includes amemristor coupled in electrical series between at least two electrodesand a thermally-insulating cladding surrounding a portion of thememristor. In this connection, the electrodes used may also be morethermally insulating than in more conventional devices. NbN is morethermally insulating than the commonly used TiN while still providingadequate electrical conductivity. Multilayer electrodes such asalternating layers of TiN and TaN can also provide good thermalinsulation and appropriate electrical conductivity. Insulating thememristor may raise its temperature when a voltage or current isapplied—such as during writing—due to Joule heating. Joule heating, alsoknown as resistive heating, may occur when heat is generated in amaterial as the result of a current passing through the material.Typically, a larger voltage creates a larger current through thematerial, which causes a larger amount of heat to be released by thematerial.

In this manner, the example memristor devices disclosed herein mayexhibit accelerated switching. In some cases, switching of a memristormay be influenced by the temperature of the memristor. Without adheringto any particular theory, processes which drive ionic and electronicmotion—including drift, thermophoresis, and diffusion—may acceleratewith increasing temperature. Thus, raising the temperature of thememristor may influence switching speed of the memristor device, which,among other features, may allow for reducing the amount of time requiredfor application of a programming bias via a voltage or current or enablethe use of lower voltages and currents over the same time period.Accordingly, increasing the switching speed may mitigate errors andimprove operation efficiency.

The present teachings address two issues. First, memristor OFF-switching(reset) is typically a slower process than ON-switching (set). Second,it is sometimes difficult to limit the ON-switching process and, as aresult, memristor elements can get permanently stuck in too conductive astate.

In accordance with the teachings herein, controlling thermal gradientsin a memristor cell may be used to increase the speed with which thestate of a memristor cell can be reset relative to set, as well as limitthe conductance of the set state. Thus, controlling the switchingprocesses in memristor memory cells may be achieved by creatingbeneficial temperature gradients during cell switching operations. By“controlled switching” is meant speeding or limiting the setting orresetting of the device's resistance state.

A memristor may include a bottom electrode, a top electrode, and anactive region disposed between the two electrodes. The active region mayhave an electrically conducting filament in an electrically insulatingmedium extending between the bottom electrode and the top electrode. Thememristor may further include a temperature gradient element forcontrolling switching.

As a result, reset times may be reduced, bringing them more into linewith set times. Overdriving of the set process may be minimized, whichmay prevent memory cells from becoming stuck in the ON state and therebymay increase endurance.

This disclosure concerns memristor memory devices where the state of thedevice is determined by the distribution of oxygen vacancies. Ingeneral, temperature gradients could be used to control the switching inany memory device where the switching process is impacted by atemperature gradient.

Three of the forces that are important in moving the vacancies duringset or reset operations are drift, diffusion, and thermophoresis.Thermophoresis is a force felt by vacancies in a temperature gradient.It tends to force them toward higher temperature regions. Normaloperation of memristor cells results in significant Joule heating. Bytailoring the thermal properties and shapes of the structures that formthe memory cell, temperature gradients can be created during the resetprocess that speed the motion of vacancies in the desired direction.Similarly, the temperature gradients that occur during a set operationcan be engineered to create forces that limit the extent of the motionof the vacancies, thereby preventing the memory cell from being driveninto a state that is too conductive and from which it is difficult toreset to a high resistance state.

The motion of vacancies during set/reset operations can be axial (i.e.along a direction perpendicular to the plane of the electrodes andswitching layer) or radial (in the plane of the switching layer andperpendicular to a conducting filament), or a combination of both. As anexample, consider the axial case, which is believed to dominate inTiO_(x)-based memristors. Here a conductive filament composed of oxygenvacancies may be driven most of the way across the switching layer (fromone electrode—the “source” electrode—to the opposite electrode—the“target” electrode) to put the cell into a low resistance state. Toreset the cell to a high resistance state, the oxygen vacancies aredriven in the opposite direction and the gap between the conductingfilament and opposite electrode is widened. By choosing electrodematerials with appropriate thermal properties relative to each other andto the switching layer and filament, the axial temperature gradient inthe region of the gap can be made large during the switching processes,with the conducting filament and/or the source electrode much hotterthan the electrode that the filament is not connected to. In most cases,the thermalization time of this system will be fast compared to theswitching time so that the thermal conductivity of the local materialsis the most important parameter in engineering a large temperaturegradient during the switching process. Otherwise, choosing materialswith appropriate heat capacity can also be important.

One could, for example, choose the non-connected electrode material (thetarget electrode) to be one that is thermally anchored, by which ismeant that it is strongly thermally connected to a larger structure (astructure with a large thermal mass) that maintains a relatively lowtemperature (the target electrode itself could have a large thermalmass). It then stays relatively cool even when the end of the filamentis relatively close to it, while the other electrode (the sourceelectrode) is chosen to have a relatively low thermal conductivity sothat it is poorly thermally anchored to the ambient environment, and mayalso have a low thermal mass. Consequently, the source electrode may behotter than the target electrode during the switching process due toheat evolved between the electrodes. Thus, portions of the conductingfilament may experience axial temperature gradients that drive oxygenvacancies toward the hotter source electrode. This may speed the resetprocess when switching back to a high resistance state. It can alsolimit how small the gap becomes during a set operation, and thereforelimits the set resistance, because the temperature gradient near the tipof the filament 16 b can become larger as the tip approaches the targetelectrode. This can occur, for example, if the switching is accomplishedwith a voltage-controlled pulse so that the power dissipated throughJoule heating increases as the gap between the filament decreases and,therefore, the resistance decreases. The greater heat released withsmaller gaps leads to higher thermal gradients.

Similar techniques can be used for switching materials where radialmotion of oxygen vacancies is important. For example, a wellthermally-anchored structure can be created around the perimeter of thememristor channel, thereby creating a radial thermal gradient duringswitching operations that tends to drive oxygen vacancies toward thecenter of the channel.

The entirety of this surrounding structure need not necessarily beelectrically insulating. It could include a thin electrically insulatinglayer surrounded by something electrically conducting. Alternatively, itcould consist entirely of electrically conducting materials that are notelectrically connected to anything else (floating). Electricallyinsulating materials often have low thermal conductivities, althoughthere are certainly exceptions that could be utilized here. Al₂O₃, forexample, has a thermal conductivity that is higher than TiO₂.

The advantages of the devices disclosed herein include increased resetspeed and self-limiting of set process. This approach may also allow oneto decrease the bias needed to reset the state of a memristor cell at adesired speed, which would decrease the power required for operation. Incases where the reset bias is currently higher than the bias requiredfor set, this would also minimize the cost of the circuits required toapply the switching biases. Furthermore, lower voltage circuits aresmaller and can enable higher bit densities.

FIGS. 1A-1B and FIGS. 2A-2B depict two different situations regardingemploying a thermal gradient to change the state of a memristor. In onecase (FIGS. 1A-1B), the thermal gradient in memristor 10 is primarilyaxial (up-down, or vertically, on the sheet), while in the other case(FIGS. 2A-2B), the thermal gradient in memristor 10′ is primarily radial(side-to-side, or horizontally, on the sheet). In this connection, itmay be appreciated that FIGS. 1A-1B and FIGS. 2A-2B each represent alimiting case in the sense that the former depicts a purely axialsituation and the latter depicts a purely radial situation. In reality,some of both may occur.

In all cases, the memristor 10, 10′ has a bottom electrode 12, a topelectrode 14, and an active region 16 sandwiched between the twoelectrodes. The active region 16 is a relatively insulating region 16 ain which a relatively conducting filament 16 b is formed. As theconducting filament 16 b extends from one electrode 12 to the other(here, from the top electrode 14 to the bottom electrode 12), a gap 16 cbetween the filament 16 b and the bottom electrode 12 is reduced and thedevice 10, 10′ becomes more conducting and assumes an “ON” state.

As the conducting filament 16 b retreats from the other electrode here,from the bottom electrode 12), the gap 16 c is increased and the device10, 10′ becomes more resistive, and assumes an “OFF” state. An “ON”state may be associated with a more conductive (less resistive) device10, 10′, with an “OFF” state associated with a less conductive (moreresistive) device. The ON state is usually associated with a value “1”,while the OFF state is usually associated with a value “O”. However, insome examples, this convention may be reversed, with an “ON” stateassociated with a value “0” and an OFF state associated with a value“1”.

FIG. 1A depicts a set process in the case where motion of oxygenvacancies (O_(V) ⁺) is primarily axial (vertical in the Figure). Theconducting filament 16 b has a region with a high concentration ofoxygen vacancies and is electrically connected more strongly to the topelectrode 14 than to the bottom electrode 12. During the set process,heat is evolved in the filament 16 b through Joule heating. Heat mayalso be evolved due to Joule heating in the gap 16 c between thefilament 16 b and the bottom electrode 12, depending upon the electricalconduction mechanism (e.g., for Poole-Frenkel conduction). If theelectrical conduction involves tunneling of electrons from the bottomelectrode 12 to the bottom of the filament 16 b, heating may also occurnear the end of the filament due to thermalization of the tunnelingelectrons. The top electrode 14 may be a relatively low thermalconductivity material (at least in the region near the filament 16 b)and the bottom electrode 12 may be a relatively higher thermalconductivity material (well-thermally anchored). The temperaturegradient ∇T is as shown in FIG. 1 and drives oxygen vacancies toward thefilament 16 b (up in the Figure). The electric field E drives positivelycharged vacancies “down” to extend the filament 16 b and decrease thedevice 10 resistance (set process). As the filament 16 b approaches thebottom electrode 12, this process is retarded due to the countervailingforce on the oxygen vacancies due to the thermal gradient ∇T, therebylimiting the set process and preventing the device 10 from being “stuckon”.

FIG. 1B depicts a reset process in the case where the motion of theoxygen vacancies is primarily axial (vertical in the Figure). Theconducting filament 16 b has a region with a high concentration ofoxygen vacancies (O_(V) ⁺) and is electrically connected more stronglyto the top electrode 14 than to the bottom electrode 12. During thereset process, heat is evolved in the filament 16 b through Jouleheating. Heat may also be evolved due to Joule heating in the gap 16 cbetween the filament 16 b and the bottom electrode 12, depending uponthe electrical conduction mechanism (e.g. for Poole-Frenkel conduction).The top electrode 14 may be a relatively low thermal conductivitymaterial (at least in the region near the filament) and the bottomelectrode 12 may be a relatively higher thermal conductivity material(well-thermally anchored). The electric field E drives positivelycharged vacancies “up” to increase the gap 16 c and thereby increase thedevice resistance (reset process). The temperature gradient ∇T is asshown and also drives oxygen vacancies toward the filament 16 b (up inFIG. 2). This results in a faster reset process.

Examples of electrode 12, 14 materials suitably employed in the practiceof the teachings herein may have thermal conductivities in the range ofa few tenths (and below) of Watts/meter/Kelvins (W·m⁻¹·K⁻¹) to singledigit and tens of W·m⁻¹·K⁻¹ to hundreds of W·m⁻¹·K⁻¹. The relativethermal conductivity may be important, and thus it is the ratio of thetwo values that may be considered. In some examples, the ratio of thetwo values may be 2 or more. Examples of lower thermal conductivityelectrode materials may include TaN (1.7 to 5 W·m⁻¹·K⁻¹) and NbN (3 to 5W/m/K). Examples of higher thermal conductivity electrode materials mayinclude TiN (10 to 20 W·m⁻¹·K⁻¹), Pt (70 W·m⁻¹·K⁻¹), W (173 W·m⁻¹·K⁻¹),and Cu (353 to 401 W·m⁻¹·K⁻¹). Various combinations of materials havingdifferent thermal conductivities may be employed, so long as there is asufficient difference between them (a ratio of at least 2).

If one were to reverse the direction of the thermal gradient in FIG. 1B,but keep the direction of the electric field the same, then one couldretard the reset process. The thermal gradient ∇T, represented by thearrow, would be in the downward direction, toward the bottom electrode12 (instead of the upward direction, as shown in FIG. 1B). This wouldtend to push the oxygen vacancies (O_(V) ⁺) down, which would be atleast partially resist the upward force on the oxygen vacancies providedby the electric field that is created by positively biasing the bottomelectrode relative to the top electrode. This would have the effect ofretarding, or slowing down, the reset process. A reversed thermalgradient could be created if the top electrode 14 and/or filament 16 bwere well thermally anchored compared to the bottom electrode 12. Notethat significant heating can occur from electrons dumping energy intothe bottom electrode 12 that tunnel across the gap 16 c. This processwould be accentuated by thermally anchoring the top electrode 14, asdescribed above.

FIG. 2A depicts a set process in the case where the motion of oxygenvacancies includes a radial component (horizontal in the Figure) indevice 10′. The conducting filament 16 b has a region with a highconcentration of oxygen vacancies (O_(V) ⁺). The conductivity of thisfilament 16 b can be increased by adding oxygen vacancies to it. Duringthe set process, heat is evolved in the filament 16 b through Jouleheating. The top electrode 14 and bottom electrode 12 are each of arelatively low thermal conductivity material (at least in the regionnear the filament 16 b) and are not well thermally grounded to rest ofworld. The filament 16 is at least partially surrounded by structures 18that have a relatively high thermal conductivity and are well thermallygrounded to the rest of the world. The temperature gradient ∇T is asshown and drives oxygen vacancies toward the filament 16 b. This resultsin a faster set process.

FIG. 2B depicts a reset process in the case where the motion of oxygenvacancies includes a radial component (horizontal in the Figure) indevice 10′. However, in this case, the thermal conductivities arereversed. That is to say, the bottom electrode 12 and top electrode 14are of a relatively high thermal conductivity (at least in the regionnear the filament 16 b), and well thermally anchored, and the structures18 are of a relatively low thermal conductivity. The heat may be evolvedin or near the filament 16 b, so a structure 18 around the perimeterwill always be cooler (unless it is heated directly). However, themagnitude of the radial thermal gradient could be changed by thermallyisolating the perimeter structure 18 rather than thermally anchoring it.Essentially, this would change the ratio of the axial temperaturegradient to the radial temperature gradient (more heat would flow to theelectrodes rather than the perimeter structure 18).

The conducting filament 16 b has a region with a high concentration ofoxygen vacancies (O_(V) ⁺). The conductivity of this filament 16 b canbe increased by adding oxygen vacancies to it. During the reset process,heat is evolved in the filament 16 b through Joule heating. The topelectrode 14 and bottom electrode 12 are of a relatively high thermalconductivity material (at least in the region near the filament 16 b)and are well thermally grounded to the rest of the world. The filament16 b is at least partially surrounded by structures 18 that are not wellthermally grounded to the rest of the world. The temperature gradient ∇Tis as shown and drives oxygen vacancies toward the filament 16 b, butnot as strongly as in the case described in FIG. 2A. This limits theextent of the reset process, preventing it from being driven too far.This configuration provides a faster reset and prevention of over set.

How any of these approaches might be used in practice would depend onwhich issues tend to be more problematic and which structure was moreeffective in mitigating the corresponding issue. The latter depends onthe nature of the conducting filament, where the heat is evolved, andhow important thermophoresis is in driving set or reset. It is expectedthat one might construct a “trial” memristor structure, test it, andthen make suitable alterations, following one of the approachesdescribed above.

FIG. 3 depicts a method for controlling switching in a memristor bymeans of a thermal gradient. The method includes providing 305 thememristor. As described above, the memristor 10, 10′ includes a bottomelectrode 12, a top electrode 14, and an active region 16 disposedtherebetween. The active region 16 may have an electrically conductingfilament 16 b in an electrically insulating material 16 a extending orpartially extending between the bottom electrode 12 and the topelectrode 14. The memristor 10, 10′ further may include a temperaturegradient element for controlling switching.

The method 300 continues by providing 310 the memristor with atemperature gradient element for controlling switching. As describedabove, heat is evolved in or near the filament 16 b (e.g., in the gap)to generate a thermal gradient between the filament and the temperaturegradient element. In this connection, it should be noted that there canbe vacancies in the gap 16 c, just not enough to make it as electricallyconducting as the portions of the filament 16 b that have a highervacancy density. This distinction may matter because motion of thesevacancies can also contribute to changing the resistive state of thememristor (by adding/subtracting from the filament 16 b and/or changingthe conductance of the gap 16 c). In the case of FIGS. 1A-1B, heatevolved in the gap will flow primarily toward the target electrode,thereby contributing to the gradient in the gap 16 c and drivingvacancies in the gap “up”. Any heat evolved in the gap 16 c will not addto the gradient above the location where it is evolved (it can onlydecrease it due to the small fraction of heat that flows “up”).

In the case of an axial thermal gradient, the temperature gradientelement may be the high thermal conductivity electrode, here, bottomelectrode 12. In other examples, the top electrode 14 may be the highthermal conductivity electrode.

In the case of radial thermal gradient, the temperature gradient elementmay be the high thermal conductivity structure(s) 18 surrounding theactive region 16.

FIG. 4 depicts a method 400 of manufacturing a memristor that includes atemperature gradient element for controlling switching. The method 400includes providing 405 the memristor. As described above, the memristorincludes a bottom electrode 12, a top electrode 14, and an active region16 disposed therebetween.

The method 400 concludes by forming 410 the memristor with the bottomelectrode having a different thermal conductivity or a different degreeof thermal anchoring than the top electrode.

FIG. 5 depicts another method 500 of manufacturing a memristor thatincludes a temperature gradient element for controlling switching. Themethod 500 includes providing 505 the memristor. As described above, thememristor includes a bottom electrode 12, a top electrode 14, and anactive region 16 disposed therebetween.

The method 500 concludes by forming 510 the memristor with a materialsurrounding the active region having a different thermal conductivitythan either the bottom electrode or the top electrode.

Advantageously, the reset speed is increased and there is self-limitingof the set process. This approach may also allow one to decrease thebias needed to reset the state of a memristor cell at a desired speed,which would decrease the power required for operation. In cases wherethe reset bias is currently higher than the bias required for set, thiswould also minimize the cost of the circuits required to apply theswitching biases. Furthermore, lower voltage circuits are smaller andcan enable higher bit densities.

What is claimed is:
 1. A memristor including a bottom electrode, a topelectrode, and an active region disposed therebetween, the active regionhaving an electrically conducting filament in an electrically insulatingmedium extending between the bottom electrode and the top electrode, thememristor further including a temperature gradient element forcontrolling switching.
 2. The memristor of claim 1 in which thetemperature gradient element is thermally anchored relative to that ofthe bottom electrode or top electrode or both that generates a thermalgradient sufficient to drive dopant species in a predetermineddirection.
 3. The memristor of claim 2, in which the temperaturegradient element is one of the top electrode and the bottom electrodeand has a different thermal anchoring relative to that of the other ofthe bottom electrode and the top electrode, thereby creating an axialtemperature gradient.
 4. The memristor of claim 3, in which a ratio ofthermal conductivity of one electrode to thermal conductivity of theother electrode is at least two.
 5. The memristor of claim 2, in whichthe temperature gradient element comprises a material surrounding theactive region and has a different thermal anchoring relative to that ofthe bottom electrode or the top electrode, thereby impacting the radialtemperature gradient.
 6. The memristor of claim 5, in which a ratio ofthermal conductivity of the temperature gradient element and the thermalconductivity of the bottom electrode or the top electrode is at least afactor of two.
 7. A method of controlling switching in a memristor, themethod including: providing a memristor having a bottom electrode, a topelectrode, and an active region disposed therebetween, the active regionhaving an electrically conducting filament in an electrically insulatingextending between the bottom electrode and the top electrode, andproviding the memristor with a temperature gradient element forcontrolling switching.
 8. The method of claim 7 in which the temperaturegradient element has a thermal anchoring relative to that of the bottomelectrode or top electrode or both that generates a thermal gradientsufficient to drive dopant species in a predetermined direction.
 9. Themethod of claim 8, in which the thermal conductivity difference betweenthe temperature gradient element and the top electrode or bottomelectrode or is at least a factor of two.
 10. The method of claim 9, inwhich the temperature gradient element is one of the top electrode andthe bottom electrode and has a different thermal conductivity relativeto that of the other of the bottom electrode and the top electrode,thereby creating an axial temperature gradient.
 11. The method of claim9, in which the temperature gradient element comprises a materialsurrounding the active region and has a different thermal conductivityrelative to that of the bottom electrode or the top electrode, therebyaltering a radial gradient.
 12. A method of manufacturing a memristorthat includes a temperature gradient element for controlling switching,the memristor further including a bottom electrode, a top electrode, andan active region disposed therebetween, the method comprising: eitherforming the memristor with the bottom electrode having a differentthermal conductivity than the top electrode, or forming the memristorwith a material surrounding the active region having a different thermalconductivity than either the bottom electrode or the top electrode. 13.The method of claim 12, in which the temperature gradient element has athermal conductivity relative to that of the bottom electrode or topelectrode or both that generates a thermal gradient sufficient to drivedopant species in a predetermined direction, and in which the thermalconductivity difference between the temperature gradient element and thetop electrode or bottom electrode or is at least a factor of two. 14.The method of claim 13, in which one of the top electrode and the bottomelectrode has a high thermal conductivity relative to the other of thebottom electrode and the top electrode, thereby creating an axialtemperature gradient.
 15. The method of claim 13, in which the materialsurrounding the active region has a high thermal conductivity materialrelative to the active region, thereby creating a radial temperaturegradient.